sched: rework of "prioritize non-migratable tasks over migratable ones"

regarding this commit: 45c01e824991b2dd0a332e19efc4901acb31209f

I think we can do it simpler. Please take a look at the patch below.

Instead of having 2 separate arrays (which is + ~800 bytes on x86_32 and
twice so on x86_64), let's add "exclusive" (the ones that are bound to
this CPU) tasks to the head of the queue and "shared" ones -- to the
end.

In case of a few newly woken up "exclusive" tasks, they are 'stacked'
(not queued as now), meaning that a task {i+1} is being placed in front
of the previously woken up task {i}. But I don't think that this
behavior may cause any realistic problems.

There are a couple of changes on top of this one.

(1) in check_preempt_curr_rt()

I don't think there is a need for the "pick_next_rt_entity(rq, &rq->rt)
!= &rq->curr->rt" check.

enqueue_task_rt(p) and check_preempt_curr_rt() are always called one
after another with rq->lock being held so the following check
"p->rt.nr_cpus_allowed == 1 && rq->curr->rt.nr_cpus_allowed != 1" should
be enough (well, just its left part) to guarantee that 'p' has been
queued in front of the 'curr'.

(2) in set_cpus_allowed_rt()

I don't thinks there is a need for requeue_task_rt() here.

Perhaps, the only case when 'requeue' (+ reschedule) might be useful is
as follows:

i) weight == 1 && cpu_isset(task_cpu(p), *new_mask)

i.e. a task is being bound to this CPU);

ii) 'p' != rq->curr

but here, 'p' has already been on this CPU for a while and was not
migrated. i.e. it's possible that 'rq->curr' would not have high chances
to be migrated right at this particular moment (although, has chance in
a bit longer term), should we allow it to be preempted.

Anyway, I think we should not perhaps make it more complex trying to
address some rare corner cases. For instance, that's why a single queue
approach would be preferable. Unless I'm missing something obvious, this
approach gives us similar functionality at lower cost.

Verified only compilation-wise.

(Almost)-Signed-off-by: Dmitry Adamushko <dmitry.adamushko@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 2 insertions, 4 deletions

diff --git a/kernel/sched.c b/kernel/sched.c
index 554de400980..cc1d558406f 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -153,8 +153,7 @@ static inline int task_has_rt_policy(struct task_struct *p)
  */
 struct rt_prio_array {
 	DECLARE_BITMAP(bitmap, MAX_RT_PRIO+1); /* include 1 bit for delimiter */
-	struct list_head xqueue[MAX_RT_PRIO]; /* exclusive queue */
-	struct list_head squeue[MAX_RT_PRIO];  /* shared queue */
+	struct list_head queue[MAX_RT_PRIO];
 };
 
 struct rt_bandwidth {
@@ -7620,8 +7619,7 @@ static void init_rt_rq(struct rt_rq *rt_rq, struct rq *rq)
 
 	array = &rt_rq->active;
 	for (i = 0; i < MAX_RT_PRIO; i++) {
-		INIT_LIST_HEAD(array->xqueue + i);
-		INIT_LIST_HEAD(array->squeue + i);
+		INIT_LIST_HEAD(array->queue + i);
 		__clear_bit(i, array->bitmap);
 	}
 	/* delimiter for bitsearch: */